Equivalent circuit of doubly-fed wind turbine generator

Abstract

The utility model relates to doubly -fed wind turbine unit technical field especially is a kind of doubly -fed wind turbine unit equivalent circuit, including rotor side converter and net side converter, the rotor side converter with the net side converter is connected through DC bus, and the net side converter includes series connection's line resistance, line inductance and gate control series capacitor.The utility model is aimed at the subsynchronous oscillation phenomenon of doubly -fed wind turbine unit, proposes to utilize gate control series capacitor to suppress it, changes system reactance by adjusting the reference value of gate control series capacitor, destroys the resonance point of the subsynchronous oscillation, and then realizes the purpose of inhibiting subsynchronous oscillation phenomenon.

Description

Technical Field

[0001] This utility model relates to the field of doubly fed wind turbine technology, and in particular to an equivalent circuit of a doubly fed wind turbine. Background Technology

[0002] Currently, numerous solutions have been proposed to suppress subsynchronous oscillations (SSO) generated by series compensation systems in doubly-fed wind farms, primarily focusing on rotor-side and grid-side converters. Power-side SSO suppression strategies concentrate on active intervention in the wind turbine control system, including controller parameter optimization and additional damping control. At the parameter optimization level, existing research aims to reduce the negative damping risk in the subsynchronous frequency band by adjusting the converter's inner-loop proportional gain and the dynamic response characteristics of the phase-locked loop. However, such methods are essentially limited to local stability adjustment, only suppressing the amplitude of a single oscillation mode and failing to address multi-mode oscillation coupling and wide-frequency instability issues. Especially when operating conditions such as grid strength and wind speed deviate from the preset optimization range, a mismatch occurs between the time-invariance of control parameters and the dynamic time-varying characteristics of the system, leading to a significant decrease in suppression effectiveness and even inducing mode migration in the static synchronous series compensator (SSSC). Therefore, solutions relying solely on parameter optimization face insufficient robustness in complex and variable real-world engineering scenarios.

[0003] To address the subsynchronous oscillation (SSO) suppression problem in doubly-fed induction generator (DFIG) wind turbines, some literature proposes comparative schemes by embedding additional damping control modules in the rotor-side converter (RSC) and grid-side converter (GSC), respectively. Simulation analysis results show that the RSC additional damping controller exhibits more significant oscillation attenuation characteristics. Some literature proposes integrating a resonant controller into the converter controller of the DFIG wind turbine system, exploring the impact of different parameters associated with the resonant controller on system stability to determine the optimal parameters for enhanced damping characteristics. Some researchers employ active damping control based on flexible AC transmission system devices on the grid side, reshaping network impedance characteristics to avoid the system resonance point and suppress SSO, such as static var compensators (SVCs), static synchronous compensators (SSCs), and unified power flow controllers. Using a static synchronous series compensator (SSSC) to increase the system's equivalent positive resistance and improving robustness through additional active resistance control, thereby achieving the goal of SSO suppression.

[0004] While existing damping solutions demonstrate some effectiveness under specific operating conditions, their strategies based on additional damping control have limitations and are difficult to adapt to complex operating scenarios such as grid intensity fluctuations and random wind speed variations. Retrofitting existing wind farms requires large-scale hardware replacements and software upgrades, resulting in high retrofitting costs and tight deadlines. Utility Model Content

[0005] In this section, as well as in the abstract and title of this application, some simplifications or omissions may be made to avoid obscuring the purpose of this section, the abstract, and the title of this application. Such simplifications or omissions shall not be used to limit the scope of this utility model.

[0006] To address the shortcomings of existing technologies, one objective of this utility model is to provide an equivalent circuit for a doubly fed wind turbine.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: an equivalent circuit for a doubly fed wind turbine generator, comprising a rotor-side converter and a grid-side converter, wherein the rotor-side converter and the grid-side converter are connected via a DC bus. The grid-side converter includes a line resistor, a line inductance, and a gated series capacitor connected in series.

[0008] As a preferred embodiment of the equivalent circuit of the doubly fed wind turbine of this utility model, the gated series capacitor is composed of a fixed capacitor connected in parallel with two reverse turn-off thyristors.

[0009] As a preferred embodiment of the equivalent circuit of the doubly fed wind turbine of this utility model, a controller is provided at both ends of the gated series capacitor, and the output terminal of the controller is connected to the gate of the thyristor.

[0010] As a preferred embodiment of the equivalent circuit of the doubly fed wind turbine of this utility model, the controller includes a main control chip and a driver chip. The GPIO0 pin of the main control chip is connected to the ANODE pin of the driver chip, the VEE pin of the driver chip is connected to the cathode of the thyristor, and the VOUT pin of the driver chip is connected to the gate of the thyristor.

[0011] As a preferred embodiment of the equivalent circuit of the doubly fed wind turbine of this utility model, the grid-side converter further includes a system AC voltage, one end of which is connected to one end of the gated series capacitor, and the other end of which is grounded.

[0012] As a preferred embodiment of the equivalent circuit of the doubly fed wind turbine of this utility model, the rotor-side converter includes a rotor voltage, one end of which is grounded and the other end of which is connected to the DC bus.

[0013] As a preferred embodiment of the equivalent circuit of the doubly fed wind turbine of this utility model, a first equivalent circuit is connected in series between the rotor voltage and the DC bus. The first equivalent circuit includes the stator winding leakage inductance and the stator winding resistance connected in series.

[0014] As a preferred embodiment of the equivalent circuit of the doubly fed wind turbine of this utility model, a second equivalent circuit is connected in series between the rotor voltage and the first equivalent circuit, and the second equivalent circuit includes the rotor winding leakage inductance and the rotor winding resistance connected in series.

[0015] In a preferred embodiment of the equivalent circuit of the doubly fed wind turbine of this invention, an equivalent resistance is connected in series between the rotor voltage and the second equivalent circuit.

[0016] As a preferred embodiment of the equivalent circuit of the doubly fed wind turbine of this utility model, an excitation inductor is provided in parallel between the first equivalent circuit and the second equivalent circuit, and one end of the excitation inductor is grounded.

[0017] The beneficial effects of the equivalent circuit of the doubly fed wind turbine in this utility model are as follows: This utility model addresses the subsynchronous oscillation phenomenon of doubly fed wind turbines by proposing to suppress it using a gated series capacitor. By adjusting the reference value of the gated series capacitor, the system reactance is changed, thereby disrupting the resonance point where subsynchronous oscillation occurs, and thus achieving the purpose of suppressing the subsynchronous oscillation phenomenon.

[0018] Gated series capacitors have unique advantages when applied to doubly-fed wind turbines: on the one hand, their power frequency compensation characteristics can partially replace traditional fixed series compensation capacitors, reducing the risk of grid resonance from the source; on the other hand, the impedance regulation capability of gated series capacitors is less affected by the grid topology and operating mode, and can adapt to the complex operating conditions of wind farms. Attached Figure Description

[0019] To more clearly illustrate the technical solutions of the embodiments of this utility model, the drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this utility model. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0020] Figure 1 This is a schematic diagram showing the connection between the rotor-side converter and the grid-side converter of this utility model.

[0021] Figure 2 This is a schematic diagram of the gated series capacitor of this utility model.

[0022] Figure 3 This is a schematic diagram of the overall structure of this utility model.

[0023] Figure 4 This is a schematic diagram showing the connection between the main control chip and the driver chip of this utility model. Detailed Implementation

[0024] To enable those skilled in the art to better understand this utility model, the present utility model will be further described in detail below with reference to specific embodiments and accompanying drawings.

[0025] The terminology used in this invention refers to those general terms currently widely used in the art in consideration of the functionality of this invention; however, these terms may vary according to the intent, precedent, or new technology of those skilled in the art. Furthermore, specific terms may be chosen by the applicant, and in such cases, their detailed meanings will be described in the detailed description of this invention. Therefore, the terminology used in this specification should not be construed as simple names, but rather based on the meaning of the terms and the overall description of this invention.

[0026] Reference Figures 1-3 This embodiment provides an equivalent circuit for a doubly fed wind turbine, including a rotor-side converter RSC and a grid-side converter GSC.

[0027] The rotor-side converter RSC and the grid-side converter GSC are connected via a DC bus M.

[0028] The doubly-fed induction generator (DFIG) wind turbine comprises two core converters located at different points along the energy transfer path. The rotor-side converter (RSC) is directly connected to the generator rotor windings, controlling the rotor current to achieve maximum wind energy tracking and torque regulation. The grid-side converter (GSC) is directly connected to the power grid, controlling the DC bus voltage stability and the grid's reactive power.

[0029] The grid-side converter GSC includes a line resistor R, a line inductance L, and a gated series capacitor GCSC connected in series. The grid-side converter GSC also includes a system AC voltage Ug, one end of which is connected to one end of the gated series capacitor GCSC, and the other end of which is grounded.

[0030] Wherein, the system AC voltage Ug is the system equivalent disturbance voltage source at the subsynchronous frequency. It represents the disturbance voltage generated by the resonance of the fixed capacitor and the line inductance L of the power grid at the subsynchronous oscillation frequency. This voltage source is related to the resonant response excited in the grid-side converter GSC circuit after the system is disturbed.

[0031] The system AC voltage Ug, along with the fixed capacitor and the line inductance L, constitutes the core excitation source of the resonant circuit. By replacing the fixed capacitor with a gated series capacitor GCSC, it directly acts on the generation stage of the system AC voltage Ug. This allows the gated series capacitor GCSC to dynamically adjust the capacitive reactance, disrupting the resonance condition and thus eliminating the excitation source of the system AC voltage Ug.

[0032] The rotor-side converter RSC includes a rotor voltage Ur, one end of which is grounded and the other end is connected to the DC bus M.

[0033] A first equivalent circuit A is connected in series between the rotor voltage Ur and the DC bus M. The first equivalent circuit A includes the stator winding leakage inductance Ls and the stator winding resistance Rs connected in series.

[0034] A second equivalent circuit B is connected in series between the rotor voltage Ur and the first equivalent circuit A. The second equivalent circuit B includes the rotor winding leakage inductance Lr and the rotor winding resistance Rr, which are connected in series.

[0035] An equivalent resistance Rrsc is connected in series between the rotor voltage Ur and the second equivalent circuit B.

[0036] A magnetizing inductor Lm is connected in parallel between the first equivalent circuit A and the second equivalent circuit B, with one end of the magnetizing inductor Lm grounded.

[0037] One end of the DC bus M is connected in sequence to the stator winding leakage inductance Ls, stator winding resistance Rs, rotor winding leakage inductance Lr, rotor winding resistance Rr, equivalent resistance Rrsc and rotor voltage Ur, and the magnetizing inductance Lm is connected in parallel between the stator winding resistance Rs and the rotor winding leakage inductance Lr.

[0038] Among them, the rotor winding leakage inductance Lr is the inductance corresponding to the part of the magnetic flux generated by the rotor winding current that is not coupled with the stator winding. The rotor winding leakage inductance Lr hinders the change of rotor current and affects the dynamic response speed of the rotor-side converter RSC. When the subsynchronous current flows through the rotor winding leakage inductance Lr, a voltage drop will be generated, which together with the rotor voltage Ur generated by the rotor-side converter RSC affects the negative resistance effect.

[0039] The magnetizing inductance Lm is the inductance corresponding to the main magnetic flux path through which the stator and rotor windings achieve magnetic coupling, reflecting the energy transfer capability. The magnetizing inductance Lm determines the magnitude of the motor's excitation current, affecting torque generation and power conversion efficiency. The secondary synchronization current can be transmitted between the stator and rotor via the magnetizing inductance Lm. The magnetizing inductance Lm has a relatively large inductive reactance, making the equivalent impedance on the rotor side approximately a negative resistance at the secondary synchronization frequency.

[0040] The stator winding leakage inductance Ls is the inductance corresponding to the portion of the magnetic flux generated by the stator winding current that fails to couple with the rotor winding. It is used to limit the rate of change of the stator current, thus affecting the power quality of the grid-connected power supply.

[0041] The rotor voltage Ur is the disturbance voltage output by the rotor-side converter RSC, which is the equivalent voltage generated by the subsynchronous disturbance current in the rotor winding through the control loop of the rotor-side converter RSC. When the subsynchronous current flows through the rotor winding, the high-gain control of the rotor-side converter RSC will output a rotor voltage Ur that is in phase with the disturbance current. This is equivalent to introducing a negative resistance in the rotor circuit. The negative resistance will inject energy into the system and promote subsynchronous oscillation.

[0042] The mechanism of subsynchronous oscillation when a doubly fed wind turbine is connected to the grid via series compensation will be explained next.

[0043] When a doubly fed wind turbine is connected to the grid via a fixed series capacitor system, for ease of analysis, the subsynchronous oscillation frequency it generates is defined as fr, the rotor frequency as ff, the equivalent resistance of the system as Req, and the rotor slip as s.

[0044] The equivalent system resistance Req = stator winding resistance Rs + rotor winding resistance Rr + equivalent resistance Rrsc + line resistance R.

[0045] The rotor winding resistance Rr and the equivalent resistance Rrsc are affected by the rotor slip s. When s < 0, the slip is negative under subsynchronous operating conditions. When Req < 0, the system exhibits negative damping.

[0046] Based on the RLC series resonance principle, i.e., inductive reactance XL = capacitive reactance Xc, we can derive Formula 1: , Formula 1 describes the series resonance condition in an electrical system. The system will resonate when the inductive reactance XL equals the capacitive reactance Xc.

[0047] The impedance of an inductor to alternating current is calculated using Formula 2: ,

[0048] The impedance of a capacitor to alternating current is calculated using Formula 3: , in, This is the secondary synchronization angular frequency. The total inductance includes the sum of the magnetizing inductance Lm, the stator winding leakage inductance Ls, and the rotor winding leakage inductance Lr. C represents the capacitance, which is the value of the series compensation capacitor. In traditional schemes, this is a fixed-value capacitor. In this invention, the fixed-value capacitor is replaced by a gated series capacitor GCSC. The equivalent capacitive reactance (Xc) of the gated series capacitor GCSC can be dynamically adjusted by the turn-off angle γ. This makes the inductive reactance XL not equal to the capacitive reactance Xc, thereby disrupting the resonance condition.

[0049] Subsynchronous angular frequency The inductive reactance and capacitive reactance are matched, i.e., the resonant point is used to trigger oscillation.

[0050] When Req < 0, the system damping is negative, exhibiting negative resistance characteristics. Therefore, the system will lose stability and oscillate due to the lack of positive damping after being disturbed. Specifically, the positive feedback loop of the rotor-side converter RSC increases the subsynchronous current in the rotor windings, strengthening the series capacitor coupling between the converter and the transmission line. This, in turn, increases the equivalent negative resistance characteristic of the rotor-side converter RSC, causing Req to become negative.

[0051] Therefore, based on the above analysis, we can derive solutions to suppress subsynchronous oscillations caused by doubly-fed induction generator (DFIG) wind turbines connected to the grid via fixed series compensation. On one hand, considering Formula 1, we can disrupt the resonance point by adjusting the system's equivalent reactance to change or eliminate the resonance point, thereby suppressing subsynchronous oscillations. On the other hand, we can increase the electrical damping at the resonance point by changing the system's equivalent resistance, making it positive, thus suppressing subsynchronous oscillations.

[0052] As the above analysis shows, the negative resistance characteristic of doubly-fed induction generator (DFIG) wind turbines connected to the grid via series compensation is the main cause of subsynchronous oscillations in the system. Since Req < 0 lacks positive damping to suppress subsynchronous oscillations, some literature proposes using an additional control strategy on the rotor-side converter's RSC to make it equivalent to a positive resistance, thereby reducing the negative resistance characteristic of the rotor-side converter's RSC and allowing Req > 0 to return to the positive value region. This would give the system positive damping characteristics, thus avoiding subsynchronous oscillations. However, in actual operation, the modification and maintenance of wind turbines face numerous difficulties. Therefore, this invention considers this from another perspective, suppressing subsynchronous resonance at its root.

[0053] Reference Figure 2 The gated series capacitor GCSC is composed of a fixed capacitor C connected in parallel with two reverse turn-off thyristors T.

[0054] Among them, gated series capacitors (GCSCs), as a new type of flexible AC transmission system (FACTS) device, have been widely used due to their simple control strategy and certain economic advantages. Each phase of a gated series capacitor GCSC consists of a fixed capacitor C connected in parallel with two reverse turn-off thyristors T, and control is achieved by changing the turn-off angle γ measured at the zero-crossing point of the line current.

[0055] A controller N is provided at both ends of the gated series capacitor GCSC, and the output of the controller N is connected to the gate of the thyristor T.

[0056] The controller N monitors system parameters in real time, dynamically adjusts the turn-off angle γ, and precisely controls the input of the gated series capacitor GCSC, effectively disrupting the resonance point, enhancing system damping, thereby stabilizing operation and avoiding subsynchronous oscillations.

[0057] Reference Figure 4The controller N includes a main control chip N1 and a driver chip N2. The GPIO0 pin of the main control chip N1 is connected to the ANODE pin of the driver chip N2. The VEE pin of the driver chip N2 is connected to the cathode K of the thyristor T. The VOUT pin of the driver chip N2 is connected to the gate G of the thyristor T.

[0058] A voltage transformer or Hall voltage sensor is connected in parallel to the transmission line where the gated series capacitor GCSC is located to proportionally convert the high voltage into a low voltage safety signal.

[0059] A current transformer or Hall effect current sensor is connected in series in the circuit to convert a large current into a small current or low voltage signal proportionally.

[0060] The sensor output signal, after being conditioned, becomes an analog voltage signal that is ultimately connected to a specific ADC input pin of the main control chip N1. The ADC input pin can be PA0, PA1, PA2, PC0, PC1, PC2, or PA3.

[0061] The PA0 pin receives the current signal from the A-phase line being tested. The PA1 pin receives the current signal from the B-phase line being tested. The PA2 pin receives the current signal from the C-phase line being tested. The PC0 pin receives the voltage signal from the A-phase line being tested. The PC1 pin receives the voltage signal from the B-phase line being tested. The PC2 pin receives the voltage signal from the C-phase line being tested. The PA3 pin receives the voltage signal from the DC bus being tested.

[0062] The main control chip N1 collects system parameters in real time, such as line current, voltage, and frequency, and calculates whether the current system state is in the risk range of subsynchronous oscillation.

[0063] The main control chip N1 calculates the required turn-off angle γ using an algorithm. This angle determines the equivalent capacitive reactance of the gated series capacitor GCSC. The turn-off angle γ value is converted into a corresponding pulse signal and output to the ANODE pin of the driver chip N2 through the GPIO0 pin.

[0064] The driver chip N2 receives the pulse signal from N1, performs level conversion and power amplification, enabling it to drive a high-power thyristor. The VEE pin is connected to the cathode K of the thyristor, providing a reference ground potential. The VOUT pin outputs a drive pulse to the gate G of the thyristor, controlling its turn-on and turn-off timing.

[0065] When the main control chip N1 detects the subsynchronous oscillation trend, the driver chip N2 rapidly reduces the turn-off angle γ, lowers the equivalent capacitive reactance, and disrupts the resonance condition.

[0066] When the system is stable, the driver chip N2 appropriately increases the turn-off angle γ to provide necessary capacitive compensation and maintain voltage stability.

[0067] By dynamically adjusting the equivalent capacitive reactance of the gated series capacitor GCSC, the total impedance of the system no longer satisfies the resonance condition of inductive reactance XL = capacitive reactance Xc.

[0068] Finally, it should be noted that the methods and devices described in detail above are merely embodiments, and those skilled in the art can modify these embodiments in different ways as long as they do not depart from the scope of this utility model.

Claims

1. An equivalent circuit for a doubly-fed wind turbine generator, characterized in that: It includes a rotor-side converter (RSC) and a grid-side converter (GSC), and the rotor-side converter (RSC) and the grid-side converter (GSC) are connected by a DC bus (M); The grid-side converter (GSC) includes a line resistor (R), a line inductance (L), and a gated series capacitor (GCSC) connected in series.

2. The equivalent circuit of the doubly-fed wind turbine as described in claim 1, characterized in that: The gated series capacitor (GCSC) consists of a fixed capacitor (C) connected in parallel with two reverse turn-off thyristors (T).

3. The equivalent circuit of the doubly-fed wind turbine as described in claim 2, characterized in that: The gated series capacitor (GCSC) is equipped with a controller (N) at both ends, and the output of the controller (N) is connected to the gate of the thyristor (T).

4. The equivalent circuit of the doubly-fed wind turbine as described in claim 3, characterized in that: The controller (N) includes a main control chip (N1) and a driver chip (N2). The GPIO0 pin of the main control chip (N1) is connected to the ANODE pin of the driver chip (N2). The VEE pin of the driver chip (N2) is connected to the cathode (K) of the thyristor (T). The VOUT pin of the driver chip (N2) is connected to the gate (G) of the thyristor (T).

5. The equivalent circuit of a doubly-fed wind turbine as described in any one of claims 1 to 4, characterized in that: The grid-side converter (GSC) also includes a system AC voltage (Ug), one end of which is connected to one end of the gated series capacitor (GCSC), and the other end of which is grounded.

6. The equivalent circuit of a doubly-fed wind turbine as described in any one of claims 1 to 4, characterized in that: The rotor-side converter (RSC) includes a rotor voltage (Ur), one end of which is grounded and the other end of which is connected to the DC bus (M).

7. The equivalent circuit of the doubly-fed wind turbine as described in claim 6, characterized in that: A first equivalent circuit (A) is connected in series between the rotor voltage (Ur) and the DC bus (M). The first equivalent circuit (A) includes the stator winding leakage inductance (Ls) and the stator winding resistance (Rs) connected in series.

8. The equivalent circuit of the doubly-fed wind turbine as described in claim 7, characterized in that: A second equivalent circuit (B) is connected in series between the rotor voltage (Ur) and the first equivalent circuit (A). The second equivalent circuit (B) includes the rotor winding leakage inductance (Lr) and the rotor winding resistance (Rr) connected in series.

9. The equivalent circuit of the doubly-fed wind turbine as described in claim 8, characterized in that: An equivalent resistance (Rrsc) is connected in series between the rotor voltage (Ur) and the second equivalent circuit (B).

10. The equivalent circuit of a doubly-fed wind turbine as described in claim 8 or 9, characterized in that: An excitation inductor (Lm) is connected in parallel between the first equivalent circuit (A) and the second equivalent circuit (B), and one end of the excitation inductor (Lm) is grounded.

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